Most conventional turbochargers include a shaft fixed to a turbine wheel, which is often referred to as a shaft and turbine wheel assembly (SWA). During assembly of a turbocharger, the shaft of a SWA is usually inserted through a bearing bore of a center housing such that the free end of the shaft can be fitted with a compressor wheel.
As operational environments of a compressor wheel, a shaft and a turbine wheel differ, materials of construction and manners of construction of these components can also differ. For example, exhaust temperatures may exceed 500 degrees C. for a diesel engine and 1000 degrees C. for a gasoline engine thereby requiring high-temperature materials for turbine wheel construction. Further, while a shaft may be made of a moderate temperature resistant metal or metal alloy turned on a lathe, a turbine wheel is typically cast using a high-temperature resistant ceramic, metal or metal alloy (e.g., consider austenitic nickel-chromium-based superalloys marketed under the brand INCONEL®, Special Metals Corporation, Huntington W. Va.).
Various techniques can be used to join a shaft to a turbine wheel. One technique is electron beam welding, which involves placing a shaft and turbine wheel in a vacuum and focusing an electron beam on a joint to be welded. Another energy beam-based technique involves one or more laser beams to deliver energy sufficient to weld components together. Other techniques include, for example, friction welding where one component is rotated and contacted with another component to generate heat that causes melting of material. In general, it is beneficial for a weld to be formed in a manner that requires minimal post-weld machining. It is also beneficial for a weld to be formed in a manner that does not introduce excessive stresses.
Overall, a SWA should be able to withstand temperature variations and centrifugal loading without experiencing significant deformation (e.g., which could alter balance, introduce noise, reduce assembly life, etc.). As described in various examples herein, various components, assemblies and techniques may improve SWA quality and turbocharger performance.